Radio receivers are known. Such receivers are constructed to receive signals on a designated frequency. As the number of transmitters and receivers has increased competition for the use of designated frequencies has increased. Because of the competition for radio spectrum efforts have been made to increase the efficiency of radio channel use. Methods of accomplishing this objective include time division multiplexing, conversion of voice and other signals into a digital format, and so forth. By increasing the data transmission rates a given radio transmitter does not need to transmit for as long a time period to transmit the same amount of information. Reduced transmission time periods allows an increased number of radios to operate on the same designated frequency.
While an increase in the information rate increases the efficiency of radio channel use, radio receivers must be constructed to receive information at the increased rate. Radio receivers, in fact, have been constructed to receive information at high data rates. These radios operate well and with small error rates under most conditions. A problem, on the other hand, arises in high speed data transmissions when there are a large number of transmitters sharing the same channel and each transmitter is only allowed to operate for short periods. Under these conditions intended receivers must quickly detect a desired transmission and synchronize to that transmission to receive the transmitted information.
All receivers operating on a given frequency must quickly decode every transmission to determine if a given transmission is intended for that receiver. An inability to detect a given transmission results in a need for a transmission to be repeated. Repeated transmissions result in reduced efficiency.
Reduced efficiency also results, on the other hand, where relatively long time periods are allowed receivers to detect and to synchronize with a transmitted signal.
In the past the detection of signals has been based upon a technique of measuring the magnitude of a received signal. The difficulty with the technique lies in the suitable selection of a detection threshold particularly in a TDM environment where the desired signals may have widely varying intensities, and/or the desired signal occurs immediately before or after another signal (perhaps undesired) of significantly different amplitude than the desired signal. In such an environment if the detection threshold is set too low, a strong interfering signal may trigger the detector; conversely, if the threshold is set too high then desired signals may go undetected. A need therefore exists for a method which can accurately detect the desired signal through other means.
Digital signal processing is known in the art. Digital techniques provide a method of gathering data about a time varying signal, possibly utilizing a computing device such as a microprocessor or digital signal processor. As is known the frequency of data sampling must be at least twice the bandwidth of the sampled signal to obtain an accurate representation of the signal. When such procedures are followed, accurate and reproducible signal data can be gathered.
The equation ##EQU1## is also well known to those familiar in the art of communication systems. As is known the terms C.sub.i and X.sub.i are digital representations of two time varying signals (C and X). Also, as is known, the left hand term in the above equation equals the right hand term (the product, C.sub.i X.sub.i, is largest) when C.sub.i .dbd.X.sub.i * (where X.sub.i * is the complex conjugate of C.sub.i).
By virtue of the above relation, the terms (C.sub.i) representing a known signal (C), can be used to identify the presence of a desired signal, X. Specifically, when the above equation becomes an equality then the signal X (represented by sampled digital values X.sub.i) is substantially identical to the signal C (represented by the values, C.sub.i). Substantial equality of values (C.sub.i .dbd.X.sub.i), in this context, identifies the presence of the desired signal, X. Identification of the desired signal through application of terms (C.sub.i) representing the known signal (C) is known in the art as applying a matched filter to the unknown signal (X). The differences between matched filters and bandpass filters are well known in the prior art (see Principles of Communication Engineering, by Wozencraft and Jacobs, Wiley, N.Y., 1965, p. 234).